1. Field of the Invention
The invention relates to a very large bandwidth amplifier circuit, and more particularly to a transimpedance amplifier with a frequency-selective overall negative feedback active at DC and low frequencies only.
2. Description of the Related Art
Currently, optical networks are operating at several gigabits/s data rate. An important component of the optical network is the Transimpedance Amplifier (TIA). At the receiving end of the optical network, a photodiode is used to convert the optical pulses into electrical current pulses. The TIA picks up these current pulses, amplifies them and converts them into voltage pulses. At such high speeds, the conventional approach of designing TIA using overall wide-band negative feedback suffers from bandwidth as well as stability problems. This is because to maintain stability with such feedback, some bandwidth has to be sacrificed, i.e., the bandwidth has to be intentionally narrowed. Additionally, while maintaining a wide bandwidth, the TIA must have low sensitivity to DC input currents and also the output must be DC offset free so as to directly couple to the next stage. To solve these DC problems, usually a large external capacitor is required. Therefore, such implementations are not suitable for high data rate designs.
The general scheme used in the prior art is shown in FIG. 1. It is basically a photodiode PD coupled between voltage supply VDD and the − input of a high frequency operational amplifier A. Amplifier A has an overall wide-band negative feedback using a resistor R. The resistor R determines the transimpedance gain of such a TIA. The inductor L, which can be external or a simulated internal one (requiring an external capacitor) provides the low-frequency roll-off. The signal OUT is the output of the operational amplifier A. The + input of amplifier A is coupled to a reference potential AGND (analog ground—usually VDD/2).
Reference is made to U.S. Pat. No. 6,639,472 (Wilson et al.), U.S. Pat. No. 6,611,174 (Sherman), U.S. Pat. No. 6,552,615 (Pavan et al.), U.S. Pat. No. 6,593,810 (Yoon), U.S. Pat. No. 6,583,671 (Chatwin), U.S. Pat. No. 6,462,327 (Ezell et al.), U.S. Pat. No. 6,275,114 (Tanji et al.), U.S. Pat. No. 6,242,732 (Rantakari), U.S. Pat. No. 6,084,478 (Mayampurath), U.S. Pat. No. 5,606,277 (Feliz), U.S. Pat. No. 5,198,658 (Rydin), U.S. Pat. No. 5,216,386 (Wyatt), U.S. Pat. No. 4,498,001 (Smoot), and U.S. Pat. No. 4,872,209 (Blanken). The present invention is different from all above cited U.S. Patents.
Clearly a Multi-Gigabit/s transimpedance amplifier is desired with wide bandwidth, improved stability, low sensitivity to DC input currents and a DC offset free output. The present invention described hereinafter satisfies all these requirements.